Combustion engines are machines that convert chemical energy stored in fuel into mechanical energy useful for generating electricity, producing thrust, or otherwise doing work. These engines typically include several cooperative sections that contribute in some way to the energy conversion process. In gas turbine engines, air discharged from a compressor section and fuel introduced from a fuel supply are mixed together and burned in a combustion section. The products of combustion are harnessed and directed through a turbine section, where they expand and turn a central rotor shaft. The rotor shaft may, in turn, be linked to devices such as an electric generator to produce electricity.
To increase efficiency, engines are typically operated near the operational limits of the engine components. For example, to maximize the amount of energy available for conversion into electricity, the products of combustion (also referred to as the working gas or working fluid) often exit the combustion section at high temperature. This elevated temperature generates a large amount of potential energy, but it also places a great deal of stress on the downstream fluid guide components, such as the blades and vanes of the turbine section.
In many gas turbine applications, working fluid remains at high temperature even after passing through the turbine section of the engine. In these cases, additional energy may be extracted from the working fluid even after it exits the turbine section. Heat recovery steam generators (HRSGs), for example, may be used to harness energy remaining in the turbine section exhaust. Such HRSGs are typically located downstream of the engine combustion and turbine sections and use heat from the working fluid leaving the turbine section to boil water, or other suitable liquid, to produce steam. The steam is then directed to an associated steam turbine, where additional energy is recovered. In this way, return on fuel consumption increases and operational efficiency is desirably raised.
While HRSGs may be used in some situations to recover extra energy from turbine section exhaust, there are difficulties associated with this arrangement. For example, energy within the working fluid may not be removed efficiently in some engine arrangements. In other settings, HRSG components fail prematurely due to exposure to working fluid having localized unacceptably-high temperatures. Accordingly, there remains a need in this field to improve HRSG performance.